1. Field of the Invention
This invention generally relates to interferometer arrangements and to mechanical systems which incorporate such interferometer arrangements for the precise measurement and positioning of related parts which have relative motion between such parts. More specifically it relates to an interferometer arrangement which utilizes a pair of spherical elements having an index of refraction substantially equal to two at a source wavelength as collimators and/or retroreflectors. In an arrangement shown, the centers of the spherical elements define known points on bodies which exhibit relative motion. Actuators which interconnect these points are mechanically positioned so that the centers of the spherical elements and the mechanical interconnection points are substantially coincident. Also, the actuators are arranged such that their longitudinal axes are substantially coincident with the optical axis of the interferometer arrangement. In this way, extremely accurate measurements are possible using known interferometer fringe detection circuits, the outputs of which may be utilized to energize the actuators so as to maintain the position of a movable body relative to a reference body or to cause the movable body to move in a desired way relative to the reference body. The arrangements shown have the advantage of permitting more complicated motion than rotation about an axis or rectilinear translation and of permitting the accuracy achievable by optical interferometry to be exploited to the utmost, without requiring any accurate mechanical guiding members such as ways or bearings. The arrangements shown also have the advantage of being able to provide absolute values of measurements rather than relative values of measurements.
2. Description of the Prior Art
Standard forms of interferometers measure the distance in optical wavelengths between two surfaces, the images of which can be superimposed by means of an optical system. For metrological purposes, however, the distance between two discrete points is often a more useful measurement. To do this with conventional interferometers requires that the two surfaces be accurately aligned and that the relationship of these surfaces to the points be determined. The necessity for alignment imposes undesirably stringent mechanical constraints if the measurement is to be made of the variable distance between two components of a mechanism. This difficulty has been overcome in the past by using retroreflectors. However, the types of retroreflectors which have been used can operate only with light incident from a limited range of directions and require the careful alignment of several optical surfaces within each retroreflector. The cube corner type, for example, requiring three reflecting planes to be set accurately perpendicularly, operates only to 30.degree. off-axis; and suffers from second order errors of optical path lengths when constructed in the usual form of a trihedral prism. The cat's-eye form, consisting of concentric convex and concave spherical mirrors, can work up to 90.degree. off-axis, but returns a hollow beam and requires accurate concentricity. In neither case is the center of reflection easy to locate accurately with respect to the structure, because of the lack of spherical symmetry.
To overcome these difficulties accurate spherical elements of optical material with a refractive index substantially equal to two at the wavelength of the light source are used as retroflectors. A spherical element with a refractive index of two has the property of bringing an incident parallel beam of light to a focus exactly on the rear surface of the sphere. The light which is internally reflected therein returns along the same path forming a collimated beam returning along the path of the incident beam. The useful return beam is not, of course, as large as the diameter of the sphere because of spherical aberrations, but these are negligible over an appreciable aperture.
Such a retroreflector consists of a single optical surface (a complete sphere) which is a particularly easy form to generate with accuracy. The center of reflection corresponds to the center of the sphere, and is thus easy to locate with precision. The complete spherical symmetry results in an ability to return light incident from any direction.
The apparatus summarized hereinabove provides an extremely accurate interferometer arrangement for measuring distances which can be combined with actuators to position a movable body relative to a reference body or to provide rather complicated motions to the movable body relative to the reference body. The arrangements shown use feedback to maintain the movable body in a desired position or utilize controlled actuation of a plurality of actuators to obtain a desired motion of the movable body relative to the reference body.
It is therefore an object of the present invention to provide a point-to-point interferometer which incorporates as retroreflectors accurate spheres of optical material with a refractive index of approximately 2.
Another object is to provide an interferometer arrangement which can be utilized either as a reflection or transmission type interferometer.
Another object is to provide an improved interferometer arrangement by eliminating the limited angular range of prior art retroreflectors.
Still another object is to provide mechanisms which incorporate such interferometers for controlling relative motion between portions of such mechanisms.
Another object is to provide an actuator in which the longitudinal axis of the actuator is coincident with the optical axis of an interferometer.